Joint synchronizer and decoder

ABSTRACT

A joint synchronizer and decoder that implements two decision aided processes, which are referred to as “decision aided candidate selection” and “decision aided synchronization and decoding.” Decision aided candidate selection may be used to select a carrier frequency offset by selecting among a number of candidates for this parameter based on an indication of decoding success. Decision aided synchronization and decoding may be used for phase tracking based on an indication of decoding success. Although these joint synchronizing and decoding techniques may be implemented together, they may also be implemented independently. The joint synchronizer and decoder may be implemented within a return channel receiver in a DVB-RCS system using turbo coding and quadrature phase shift key (QPSK) data modulation.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional patent applicationSer. No. 60/502,283 entitled “Joint Synchronization and Decoding ofBurst Signals” filed on Sep. 12, 2003, the subject matter of which isincorporated by reference. This application is also related to andincorporates by reference U.S. patent application Ser. No.______entitled “Hybrid Frequency Offset Estimator,” which is commonly ownedand concurrently filed with the present application.

TECHNICAL FIELD

The present invention is directed generally to digital signal processingtechniques for burst-mode data signals having short observation lengthsin an environment experiencing a low signal-to-noise ratio (SNR). Theinvention is more particularly but not exclusively directed to a jointsynchronizer and decoder (JSD) adapted for use in burst-mode phase shiftkey (PSK) modulation systems based on the Digital-Video Broadcast ReturnChannel via Satellite (DVB-RCS) standard.

BACKGROUND OF THE INVENTION

The use of satellites for the transmission of voice and datacommunication signals has greatly expanded over the past decade, andsatellite links are now routinely used and relied upon for global,almost instant, bi-directional communications. Rapid improvements insatellite communications have produced beneficial impacts on manysegments of society from business and commercial applications toconsumer products. For example, residences throughout the world nowreceive news and entertainment broadcasts via satellite, in nearlyreal-time from sites in almost any country.

Burst-mode communication techniques and the related synchronizationtechniques are frequently used in satellite voice and datatransmissions. These systems typically employ multiple transmitters thatsend “bursts” of transmissions to a receiver. Bursts from the differenttransmitters are coordinated in time and frequency such that eachtransmitter can communicate with the receiver without interfering witheach other. In one type of burst signal communication, Time DivisionMultiple Access (TDMA) allows multiple users to share a single carrierwave using time-division multiplexing (TDM) to transmit multiple signalson that carrier wave. TDM divides the carrier wave into time slots andthen allocates those time slots to the different data signals.Effectively, each of the data signals takes turns accessing the carrierwave, thereby allowing a single carrier wave to carry multiplesimultaneous data transmission. A Multi-Frequency Time Division MultipleAccess (MF-TDMA) receiver simultaneously receives TDMA signals onseveral different carrier frequencies. In the MF-TDMA data transmissionscheme, any user can potentially transmit data at any frequency at anytime. The actual time slot and frequency allocation to each user isbased on capacity requests submitted by the terminal.

Within the TDMA bursts specified in DVB-RCS Standard, the carrier signalis modulated by data symbols in which a phase characteristic of thesymbol represents the data. This type of modulation technique is knownas “phase shift keying” data modulation. In general, each symbol can berepresented as a phasor in which the phase state of the symbol at thecorrect detection instant, or the relative change in phase from symbolto symbol, represents data. This data, in turn, can be expressed as acorresponding bit or combination of bits in which the number of bitscorresponds to the number of possible phase states used for datamodulation. For example, in binary phase shift keying (BPSK), eachsymbol may have one of two phase states (i.e., 0, π). Each BPSK symbolcan therefore represent a single binary digit (bit). In quadrature phaseshift keying (QPSK), each symbol may have one of four phase states(i.e., 0, π/2, π and 3π/2). Each QPSK symbol can therefore represent twobinary digits. In the general “M” phase shift keying (MPSK), each symbolmay have “m” phase states. Each MPSK symbol can therefore represent “n”binary digits, where M=2^(n).

Typically, MF-TDMA signals are de-multiplexed and re-arranged to form asignal equivalent to a single carrier. The single-carrier data signal isthen demodulated to recover the underlying data. The demultiplexing anddemodulation steps are well known in the field of communications and arethe subject of extensive research and development to improvetransmission speeds, bandwidth, accuracy, and reliability.

Several satellite data transmission standards have been adopted toharmonize the transmission and reception of satellite communicationsbroadcasts. One known standard adopted in the broadcast of Digital VideoBroadcast (DVB) signals is the Digital Video Broadcast by Satellite(DVB-S). Standard EN 300 421 of the ETSI (European TelecommunicationsStandards Institute). This standard relates to DVB services andtransparent satellite communication systems to provide DVB-S servicesdirectly to the user through an integrated receiver/decoder device thatis located in the user's home. The versatility of DVB-S in multiplexingpermits the use of a transmission capacity encompassing a variety oftelevision service configurations, including sound and data services.

The components of the DVB-S services are transmitted on a TDM carrierwave. For more information on the DVB-S standard, please refer to ETSIpublication EN 300 421 V1.1.2 (1997-98), entitled “Digital VideoBroadcasting (DVB); Framing Structure, Channel Coding and Modulation for11/12 GHz Satellite Services,” the subject matter of which is herebyincorporated herein by reference.

Satellite broadcasts are also increasingly used for instantaneoustwo-way audio, video, and data communications. Accordingly, recentattention has been given to the demand for making satellitecommunications interactive so that recipients of the broadcast can alsocommunicate back to the origin of the transmission. For example,satellite communications can be used to provide Internet connectionsbetween different users. In an effort to establish unifiedbi-directional satellite communications, the digital video broadcastwith return channel by satellite (DVB-RCS) standard has been enacted bythe ETSI.

The DVB-RCS standard relates to interaction channels on a satellitedistribution system. The purpose of this standard is to provide basicspecifications for providing interaction channels for interactivenetworks based on geostationary (GEO) satellites that incorporate returnchannel satellite terminals (RCST). The DVB-RCS standard facilitates theuse of RCSTs for domestic installations both individual and collectivetypes. The DVB-RCS standard likewise supports the connection of theterminals with home data networks, and can be applied to all frequencybands allocated to GEO satellite services. For more information on theDVB-RCS standard, refer to ETSI publication, EN 301 790 v.1.3.1, dated2003-03, entitled “Digital Video Broadcasting (DVB); Interaction Channelfor Satellite Distribution Systems,” the subject matter of which ishereby incorporated herein by reference.

Satellite communication systems operating under the DVB-RCS standard canexchange data using a variety of network and Internet technologies. Forexample, the DVB-RCS standard accommodates Asynchronous Transfer Mode(ATM) technology for transferring data in cells or packets of a fixedsize. The data packet used with ATM is relatively small compared topackets used with older technologies. The small, constant packet sizeallows ATM equipment to transmit video, audio, and computer data overthe same network, and helps assure fairness. ATM creates a fixedchannel, or route, between two points whenever data transfer begins,unlike TCP/IP that divides messages into packets that can each take adifferent route from source to destination. The DVB-RCS standard mayalso be used to transmit MPEGs (Moving Picture Experts Group), a familyof digital video compression standards and file formats that achievehigh compression rate by storing only the changes from one frame toanother, instead of each entire frame.

Loss of bursts (or packets), as measured by packet loss ratio, is a mainperformance criterion under DVB-RCS. Due to a long propagation delay ingeostationary satellite communications, the packet loss ratio should below in order to avoid performance degradation at higher network layers.The human senses are generally tolerant of slight variations, so for thetransmission of video and sound broadcasts, as defined by DVB-RCS, thepacket loss ratios is preferably in the order of 1×10⁻⁵, so that lessthan one packet is lost per hundred-thousand burst signals. For morestringent types of data transmissions, a lower packet loss may beneeded, such as 1×10⁻⁷, where less than 1 packet is lost per ten millionpacket transmissions.

One way to decrease the packet loss is to increase the signaltransmission strength or effective isotropic radiated Power (EIRP) ofthe transmitter, thereby increasing the signal-to-noise ratio at thereceiver. Improvements in the signal-to-noise ratio are desirablebecause, as provided by Shannon's theorem, the ultimate theoreticallimit to the data transmission transfer rate on a communications channelis directly proportional the signal-to-noise ratio of that channel.Consequently, increasing the power transmitted on the return channel canoften be a solution to provide adequately reliable communications.However, increasing the transmission power of the receiver unacceptablyincreases the cost of the receiver. Accordingly, there is a current needfor a demodulation technology that allows reliable communications over alow-power burst-mode signal experiencing a low signal-to-noise ratio.More specifically, there is a current need for a demodulation technologythat allows for sufficiently low packet loss rates for transmission in aDVB-RCS system while maintaining or even reducing current terminaltransmission power levels in order to minimize the cost of userterminals.

Another important aspect of DVB-RCS system application is the provisionof services at K_(a)-band frequencies (e.g., 30 GHz uplink fromterminals to satellite). The cost of the user terminal plays a majorrole in the business model of this type of services. RF components ofthe terminals are costly. Less expensive RF components result in tighterlink budget on the uplink from the terminal to satellite. For thisapplication, the use of very power efficient modems is essential inorder to maintain acceptable level of system availability.

Thus, there exist a further need for a demodulating technology thatallows for sufficiently low packet loss ratio for DVB-RCS transmissionswith a higher power efficiency. As suggested above, numerous technicaland physical problems complicate the synchronization in satellitecommunications. For instance, the synchronization may be difficult wherethe transmitter and receiver are moving relative to each other.Specifically, when a burst-mode communications transmitter is on or nearthe earth and the intended receiver is in a satellite (or when asatellite transmits to the terrestrial receiver), the spatial locationsand the relative velocities of the transmitter and receiver change overtime. The change in spatial location causes the propagation path lengthand the signal propagation time to change, and the change in relativevelocities causes a Doppler frequency to change the frequency of theburst-mode signal when it is received at the intended receiver. As aconsequence, the burst-mode signals, originally transmitted at fixedintervals, arrive at varying time intervals. Furthermore, varyingweather conditions, such as clouds and rain, also affect thecommunication signals. There is also certain level of inherent carrierfrequency uncertainty at the transmitter output. Overall, these andother conditions cause carrier frequency offset in the burst-modecommunications.

These issues are particularly present in DVB-RCS communications. At aDVB-RCS transmitter output, a 30 GHz carrier will generally appear withsome carrier frequency offset f_(o), or residual error, so that carrierfrequency (f_(c))=30 GHz±f_(o). As suggested above, contributors to thecarrier frequency offset include movement of the satellite which createsa satellite Doppler effect, uncertainty due to the satellite'stransponder, uncertainty or changes at the transmitter as to the exactcarrier frequency, and length and atmospheric conditions en route. Theseand other contributors in the system deviate the carrier off its nominalvalue and synchronization is performed to correct for the carrierfrequency offset and get the carrier back to its nominal or basebandstate. In a sense, synchronization is a fine-tuning value for bestreceiver performance.

Accordingly, the synchronization process generally includes signaldetection, finding the right timing (i.e., the symbol timing), findingthe carrier frequency offset, and tracking the carrier phase. After theright combination of these factors is determined, thereby enablingsynchronization, the data signal is demodulated from the carrier waveand the encoded symbols represented by the data signal is passed to adecoder to extract and deliver the payload data in the burst.

In synchronous digital transmission, information is conveyed byuniformly spaced pulses and the function of any receiver is to isolatethese pulses as accurately as possible. However, the received signal hasundergone changes during transmission due to the noisy nature of thetransmission channel, resulting in signal distortion, such as carrierfrequency offset and phase noise. Complete estimation and removal ofthese sources of signal distortion is necessary prior to data detection.A proper synchronization of a burst signal is needed to decode thetransmitted data, and this process typically involves theidentification, estimation and removal of these sources of signaldistortion.

Conventional synchronization methods usually operate at relatively highsignal-to-noise ratio that allow reliable synchronization. In addition,conventional burst signal demodulation techniques performsynchronization and then the decoding in a serial manner. For thisreason, a receiver generally includes a cascade of receiving filter,synchronization, and decoding to process received burst signals. Sincethe return channel is operating at a very low signal-to-noise ratio,carrier synchronization using this traditional approach cannot alonegive the right signal constellation. The currently known techniques forimproving synchronization estimations generally rely on certainthreshold criteria, such as having a sufficiently high signal-to-noiseratio, to determine the points where the synchronization problem occurs.When falling below the threshold value, the conventional algorithms failto operate properly. Much of the current research and development isdirected toward developing techniques to reduce this threshold level.

Theoretical studies can be conducted on existing estimators to determinetheir fundamental performance level, or their baseline, and this type ofmeasurement may give some sort of theoretical bound for the performanceof known techniques and algorithms used for parameter estimation. Thetheoretical limits to the performance of existing estimator designs(i.e., the performance under ideal situations) are inadequate to meetthe needs of a faint burst-mode signals transmitted at a low powerlevel, such as a return link channel in a DVB-RCS system.

As a result, there is an on-going need for techniques and systems fordetecting and correcting for signal distortion, such as the carrierfrequency offset and phase noise, that typically affect a burst-modesatellite communication system experiencing a low signal-to-noise ratio.

SUMMARY OF THE INVENTION

The present invention meets the needs described above in a jointsynchronizer and decoder (JSD) that elects among multiple candidates fora signal distortion parameter based on a measure of decoding success.This process, which is referred to as “decision aided candidateselection,” increases the likelihood of selecting the correct estimateof the signal distortion parameter when the signal-to-noise ratiodeclines to the point where the noise may mask the signal distortionparameter. As a result, a receiver utilizing this technology canidentify the correct signal distortion parameter, and thereby maintainsynchronization, at a lower signal-to-noise ratio than a conventionalreceiver. This, in turn, allows the transmitter to meet a required dataloss standard at a lower power level and cost, which is particularlyadvantageous in a system with price sensitive transmitters such asreturn channel transmitters in a satellite communication systemsoperating under the DBV-RCS standard.

In a particular embodiment, one source of signal distortion is carrierfrequency offset, and the carrier frequency offset candidates representseveral of the largest local maxima values in a modulation-freefrequency spectrum of the input signal. In this case, selecting amongmultiple candidates for the carrier frequency offset increases thelikelihood of selecting the correct carrier frequency offset when thesignal-to-noise ratio declines to the point where the largest frequencyin the modulation-free frequency spectrum may correspond to noise,rather than to the correct carrier frequency offset. But the JSD itselfis independent of the source of estimates of the signal distortion, andmay be used to process multiple candidates for sources of signaldistortion other than carrier frequency offset.

In the phase tracking embodiment described above, the JSD may alsoimplement a feedback loop in the decoding and phase tracking processreferred to as “decision directed phase tracking.” In this process, anindicator of decoding success is used as a feedback parameter in thephase tracking process. This allows the carrier phase to be adjustediteratively through a feedback loop in response to measured success ofthe decoding process. This invention represents a major improvement insynchronization technology for burst-mode communication systems, whichagain allows the transmitter to meet a required data loss standard at alower the power level and cost.

Generally described, the invention may be implemented as a JSD thatoperates on an input signal modulated with encoded data. The JSDincludes a number of channel observation modifiers that each receives anassociated candidate signal distortion indicator and channelobservations associated with the input signal. Each channel observationmodifier also produces modified channel observations including theencoded data based on the associated candidate signal distortionindicator and the channel observations. The JSD also includes asynchronization processor associated with each channel observationmodifier. Each synchronization processor receives modified channelobservations from its associated channel observation modifier,synchronizes to the modified channel observations received from itsassociated channel observation modifier, and produces corrected channelobservations. The JSD also includes a partial decoder associated witheach synchronization processor. Each partial decoder partially decodesthe encoded data embedded within the corrected channel observationsreceived from its associated synchronization processor and produces anassociated indication of decoding success. This allows a branch selectorto select a signal distortion indicator for decode processing based on acomparison of the indications of decoding success received from thepartial decoders.

The invention may also be practiced as a JSD that includes an iterativeprocessor that receives and synchronizes to channel observationsassociated with an input signal. The iterative processor includes adecoder that decodes the synchronized channel observations received froma synchronization processor and produces an associated indication ofdecoding success. The iterative processor also includes a feedback loopthat supplies the indication of decoding success to the synchronizationprocessor. This allows the synchronization processor to adjust itssynchronization to the channel observations based on the indication ofdecoding success. The iterative processor may also include a softdecision aided synchronization processor operative for computing asynchronization parameter based on the indication of decoding success.The soft decision aided synchronization processor also computescorrected channel observations based on the input channel observationsand the synchronization parameter.

In particular, the input signal to the JSD may include burst-modemulti-frequency time division multiplexed (MF-TDMA) data in which thedata signal is encoded using a quadrature phase shift key (QPSK)modulation technique in accordance with the digital video broadcastreturn channel via satellite (DVB-RCS) standard. The JSD may alsoinclude a channel observer for processing the input signal to producethe channel observations. More specifically, the channel observer mayinclude a front-end analog processor that amplifies the input signal,down-converts the amplified input signal to obtain an intermediatefrequency signal, and filters the intermediate frequency signal. Thechannel observer may also include a digital preliminary processor thatdigitizes a representation of the input signal to obtain a digitalMF-TDMA signal, demultiplexes the MF-TDMA signal to obtain aconcatenated single-channel TDMA signal, and detects burst-mode datapackets within the single-channel TDMA signal to obtain the channelobservations.

In addition, the encoded data typically includes a number of symbols,the indication of decoding success typically includes a log likelihoodratio (LLR) produced by the partial decoder for each symbol, and thebranch selector typically selects a signal distortion indicator based onthe highest average log likelihood ratio for each symbol over a commonrange of the symbols. These symbols typically have phase componentsmodulated by the encoded data.

In a particular embodiment configured for correction of carrierfrequency and phase offsets, the JSD includes a number of frequencycorrectors. Each frequency corrector receives an associated carrierfrequency offset candidate and channel observations associated with theinput signal. Each frequency corrector also produces modified. channelobservations including the encoded data based on the associated carrierfrequency offset candidate and the channel observations. The JSD alsoincludes an initial phase estimator and corrector configured toinitialize the modified channel observations to an initial phase.

This embodiment of the JSD also includes a phase tracking moduleassociated with each frequency corrector. Each phase tracking modulesynchronizes to the initialized and modified channel observationsreceived from its associated initial phase estimator and corrector toproduce phase corrected channel observations. The JSD also includes apartial decoder associated with each phase tracking module. Each partialdecoder partially decodes the encoded data embedded within the phasecorrected channel observations received from its associated phasetracking module and produces an associated indication of decodingsuccess. The JSD also includes a branch selector that selects a carrierfrequency offset candidate for decode processing based on a comparisonof the indications of decoding success received from the partialdecoders.

For this example, the invention may also be practiced as a JSD thatincludes a decoder assisted phase tracker. This module receives andsynchronizes to channel observations associated with the input signaland produces phase corrected channel observations. The JSD also includesa decoder that decodes the phase corrected channel observations receivedfrom the decoder assisted phase tracker and produces an associatedindication of decoding success. The JSD also includes a feedback loopthat supplies the indication of decoding success to the decoder assistedphase tracker. This allows the decoder assisted phase tracker to adjustits synchronization to the channel observations based on the indicationof decoding success.

In this example, the indication of decoding success typically includes alog likelihood ratio produced by the decoder. In addition, the encodeddata typically defines systematic bits representing unknown data andparity bits derived from the systematic bits. In this case, the decoderassisted phase tracker may include a soft decision aided phase trackerthat computes a phase tracking parameter based on the systematic bitsand the indication of decoding success. The soft decision aided phasetracker may also compute corrected channel observations based on thesystematic bits and the phase tracking parameter. The decoder assistedphase tracker may also include a phase tracking module that receives thephase tracking parameter from the soft decision aided phase tracker andcomputes a corrected channel observations based on the parity bits andthe phase tracking parameter. In this case, the decoder assisted phasetracker also includes a switch that feeds the systematic bits to thesoft decision aided phase tracker. The switch also feeds the parity bitsto the phase tracking module.

The phase tracking module also typically includes a feedback loop thatincludes a phase tracking parameter associated with the correctedchannel observations. In addition, the soft decision aided phase trackertypically computes the phase tracking parameter by removing modulationfrom the channel observations, obtaining a resultant signal by weightingthe modulation-free channel observations by a representation of theindication of decoding success, and computing a sliding average of theresultant signal.

Other features and advantages of the present invention will be readilyappreciated upon review of the following detailed description when takenin conjunction with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a satellite communication system in whichthe present invention may be deployed.

FIG. 2 is a conceptual drawing of the prior art showing a burst-modedata packet illustrating quadrature phase shift keying as a datamodulation technique.

FIG. 3 is a conceptual drawing of the prior art showing data symbolsillustrating the signal distortion effect of a carrier frequency offset.

FIG. 4 is a conceptual drawing of the prior art showing a burst-modedata packet illustrating a measure for carrier frequency offset.

FIG. 5 is a logic flow diagram illustrating a method for operating ageneralized joint synchronizer and decoder.

FIG. 6 is a logic flow diagram illustrating a method for operating ajoint synchronizer and decoder implementing decision aided carrierfrequency offset estimation and decision aided phase tracking.

FIG. 7 is a block diagram of a return channel receiver in a satellitecommunication system including a joint synchronizer and decoder.

FIG. 8 is a block diagram of a prior art channel observer in a returnchannel receiver in a satellite communication system.

FIG. 9 is a block diagram of a prior art front-end analog processor in areturn channel receiver in a satellite communication system.

FIG. 10 is a block diagram of a prior art digital preliminary processorin a return channel receiver in a satellite communication system.

FIG. 11 is a block diagram of a generalized joint synchronizer anddecoder.

FIG. 12 is a block diagram of a joint synchronizer and decoderconfigured to implement decision aided carrier frequency offsetestimation and decision aided phase tracking.

FIG. 13 is a block diagram of a phase tracking module for the jointsynchronizer and decoder.

FIG. 14 is a block diagram of a partial decoder for the jointsynchronizer and decoder.

FIG. 15 is a block diagram of an iterative processor for the jointsynchronizer and decoder.

FIG. 16 is a block diagram of a decoder assisted phase tracker for theiterative processor of the joint synchronizer and decoder.

FIG. 17 is a block diagram of a phase tracking module for the decoderassisted phase tracker.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The main objective of the joint synchronizer and decoder (JDS) describedbelow is to improve the power efficiency of burst-mode transmission ofencoded data. Recent advancement in error correction coding techniquesused over communication channels has made it possible to lower thetransmitted power requirement to achieve a target performance for agiven channel condition. Using schemes such as iterative decoding ofturbo codes and Low Density Parity Check (LDPC) codes, the powerrequirements approach theoretical lower bounds established by channelcapacity.

In a conventional coherent detection and decoding of encoded signal, itis essential to correct the channel observation for signal distortion,such as inter-symbol interference, carrier frequency offset, and phasenoise prior to signal decoding. In particular, for burst-mode datatransmission over satellite, the lack of accurate carrier frequency andphase synchronization can cause performance degradations that overshadowthe strength of the error correction techniques. By using the improvederror correcting technique of the JSD, acceptable data loss rates cantypically be achieved at lower a signal-to-noise ratio than thethreshold of conventional carrier synchronization circuits.

The rationale for the decoder assisted frequency correction is to allowthe production of high quality parameter estimates at a lowsignal-to-noise ratio. A parameter that plays important role insynchronization is the carrier frequency offset. It is essential toobtain an accurate estimate of the carrier frequency offset and correctthe channel observations accordingly before actual signal decoding. Forthe JSD described below, it is assumed that more than one candidate forthe carrier frequency offset is available. It should be noted that asingle estimate based on conventional estimation rules that ignore theunderlying coding structure are occasionally far from the true value.This type of error in estimation is sometimes referred to as an“outlier.” Unfortunately, the probability of encountering an outlierincreases when the signal-to-noise ratio decreases. The JSD takesadvantage of the recognition that it is highly likely that the rightestimate can be found among a fairly small number of candidates.

The proposed JSD takes advantage of the log-likelihood ratios (LLR)output from the soft-input-soft-output decoding algorithms asindications of decoding success. That is, the soft decision outputs (LLRvalues) of each decoder are used to establish a measure of likelihood ofdecoding success, so that one branch exhibiting the best indication ofdecoding success based on partial decoding can be selected for furtherprocessing. The mean absolute value of the soft decisions (LLR values)is used as a measure of merit of each branch. The soft decision valuesare computed for both systematic and parity bits. A branch with a smallfrequency estimate error is more likely to have a high averagesoft-decision value. Based on simulation results, one or two iterationsof decoding in each branch is adequate to establish a good likelihoodmeasure.

This JSD also implements methods for iterative improvement of channelobservation based on an estimate of the carrier phase within eachiteration loop. In order to modify the phase estimates, the softdecision values provided by the decoders are used to estimate andcorrect the phase variation over a block of channel observation.

More specifically described, the JSD typically implements two decisionaided processes, which are referred to as “decision aided candidateselection” and “decision aided synchronization and decoding.” Forexample, “decision aided candidate selection” may be used to select acarrier frequency offset by selecting among a number of candidates forthis parameter based on an indication of decoding success. Similarly,“decision aided synchronization and decoding” may be used for phasetracking based on an indication of decoding success. Although thesejoint synchronizing and decoding techniques may be implemented together,they may also be implemented independently.

The JSD shown in the accompanying figures and described below as a jointsynchronizer and decoder within a return channel receiver in a DVB-RCSsystem using turbo coding and quadrature phase shift keying (QPSK) datamodulation. In this system, the JSD is employed to track and removesignal distortion caused by carrier frequency offset so that thisparticular source of signal distortion can be removed to improve theability of the receiver to maintain synchronization in lowsignal-to-noise conditions. This, in turn, allows the receiver to meettypical DVB-RCS data loss requirements without increasing thetransmission power of the return channel transmitter. Of course, carrierfrequency offset is an important source of signal distortion in anycommunication system using a phase shift keying modulation techniquebecause the carrier frequency offset directly influences the phasecharacteristic of the modulating signal. Nevertheless, is should beappreciated that the invention may be applied to the identification andremoval of other types of signal distortion that may be more significantin other types of communications systems.

It should also be appreciated that the JSD may be applied to track andremove other types of signal distortion, such as phase noise or thecombination of phase noise and carrier frequency offset, in any type ofsuitable device, such as a return channel receiver operating inaccordance with the DVB-RCS standard. Accordingly, it should beunderstood that carrier frequency offset correction is one importantapplication of the JSD, but that it may also be used to track and removeother types of signal distortion, and may be used for modulation systemsother than phase shift keying.

Similarly, the JSD is described below in the context of a return channelreceiver operating in accordance with the DVB-RCS standard, but mayoperate under other standards. Furthermore, the JSD is shown asoperating in the return channel receiver of a ground-station hub ofwithin a “bent pipe” satellite communication system, which is animportant application of the invention. But the invention may also bedeployed as on board processing (OBP) on the satellite or othercommunication platform, such as an airplane, missile or spacecraft.Embodiments of the invention may also be deployed in aterminal-to-terminal “mesh” communication system, or in any othersuitable type of communication system.

The DVB-RCS standard relies on MF-TDMA burst-mode, packetizedcommunications using quadrature phase shift key (QPSK) data modulation.Within this system, individual data packets are turbo decoded andinclude known symbols in a preamble and unknown data in a payload. Atypical preamble length is 48 symbols. A typical length of the unknownpayload portion of the data packet is 440 symbols. It should beunderstood that these parameters may all be varied for embodiments ofthe invention utilizing different communication techniques. Inparticular, known symbols, also referred to as “pilot” symbols, need notbe located with a data packet preamble, but may be located anywherewithin, and may be distributed throughout, the data packet.

At present, an important purpose for deploying the JSD is to minimizethe required return channel transmission power required to maintainacceptable data loss levels for a satellite communications systemoperating under the DVB-RCS standard. Nevertheless, by tracking andremoving carrier frequency offset, the invention improves communicationsynchronization even in situations when increasing the return channeltransmission power alone may not be successful. This situation mayoccur, for example, during rain fade conditions or other circumstancesthat increase or cause fluctuations in the carrier frequency offset.

In addition, signal distortion indicators, such as carrier frequencyoffset candidates, processed by embodiments of the present invention arepreferably received from a particular embodiment of a signal distortionestimator as described in U.S. patent application Ser. No. ______entitled “Hybrid Frequency Offset Estimator,” which is incorporatedherein by reference. Nevertheless, it should be appreciated that thesignal distortion indicators could be received from any other suitabletype of device. Further, the signal distortion estimator, such as ahybrid frequency offset estimator, and the JSD may be integrated into asingle piece of equipment or they may be distributed. For example, thesignal distortion estimator may be located on a satellite ason-board-processing (OBP) equipment, whereas the JSD may be located inground-based receiver configured as customer premises equipment (CPE).Of course, any other division of data processing functionality may bedeployed to suit a particular application.

It should also be appreciated that carrier phase and frequencysynchronization is but one kind of synchronization, and that turbodecoding is but one kind of decoding scheme that may be applied to burstsignals. Thus, the particular architecture described, while presented asapplied to a DVB-RCS system using these technologies, is not limited toDVB-RCS and has application in other systems faced with the same orsimilar types of problems. Also, the invention can be applied to anytype of iterative decoding principle but is not limited to turbodecoding nor to any particular type of turbo decoding. The methoddescribed can be used for different types of coding; likewise, while theembodiment discussed is that of a carrier synchronization, this again isbut one part of “synchronization,” and the idea can also be generalizedso that for any particular part of synchronization that is lacking thenecessary performance, the joint collaboration between decoding andsynchronization of this invention can be equally applied.

Also, regardless of the type of synchronization and signals beingdecoded, the method taught does apply to burst-mode signals in everyinstance. For example, an embodiment of the invention may be applied toa continuous mode of data transmission. The principles of the inventiontaught herein with respect to burst-mode signals may also findapplication to other types of transmissions and the described method andstructure is not to be taken as necessarily limiting in this regard.Nevertheless, it should also be appreciated that the problem sought tobe overcome by the invention occurs when there are very shortobservations of the signal, which occurs with a TDMA burst-mode type ofsignal. This is when joint synchronization and decoding come into thepicture most acutely.

As shown in the embodiments described below, the carrier frequencyoffset estimator module and the JSD module conveniently interface at theoutput of the frequency offset estimator module, which typically outputsseveral candidate values for the carrier frequency offset. The number ofcandidates produced by the carrier frequency offset estimator could beany number the designer chooses, realizing that the system complexitygrows with an increase in this number. Three candidates are used in theexemplary embodiments. However, any number of candidates could produceddepending on the particular application of the invention.

Referring now to the figures, in which like numerals refer to likeelements throughout the several figures, FIG. 1 is a block diagram of a“bent pipe” satellite communication system 10 in which the presentinvention may be deployed. This type of satellite communication systemincludes a satellite 20, a number of ground station hubs represented bythe hub 30 and a larger number of end-user terminals represented by theterminal 40. For example, this simplified configuration is typical ofsatellite-based broadcast television systems in which the hub 30broadcasts television signals to the terminal 40 under the DVB-Sstandard. As noted previously, it is also advantageous to use thisinfrastructure for bidirectional communications under the DVB-RCSstandard, for example to implement interactive television or Internetservice for the receivers. For this application, the terminal 40typically sends return channel communications 50 back to the hub 30,which includes a return channel receiver 60 configured to receive anddecode these communications. The invention may be embodied within ademodulator 62, which typically includes a signal distortion estimatorsuch as a carrier frequency offset estimator, deployed within the returnchannel receiver 60.

The DVB-RCS standard specifies the characteristics of the return channelcommunications 50, which are in multi-frequency time division multipleaccess (MF-TDMA) burst-mode packet format using quadrature phase shiftkey (QPSK). data modulation. The encoded data is preferably decodedusing turbo decoding techniques, which are well known in the field ofsatellite communications. In this system, the present invention may beembodied as a frequency offset estimator within the return channelreceiver 60. The challenge is to reliably determine the carrierfrequency offset in return channel signal 50, and thereby allow thereturn channel receiver 60 maintain synchronism with the return channel.

In particular, the objective includes meting the typical DVB-RCS packetloss criteria of 1×10⁻⁵ (less than one packet is lost perhundred-thousand burst signals) and 1×10⁻⁷ (less than 1 packet is lostper ten million packet transmissions), as appropriate, without having toincrease the return channel transmission power of the terminal 40. Thisobjective becomes increasingly challenging as the signal-to-noise ratiodecreases (i.e., the return channel signal 50 becomes increasinglyfaint). The faint return signal problem can be compounded whenconditions cause the carrier frequency offset fluctuate, which canoccur, for example, during fluctuating cloud and rain fade conditions.

FIG. 2 is a conceptual drawing of a burst-mode data packet 70illustrating the quadrature phase shift keying (QPSK) data modulationtechnique specified by the DVB-RCS standard. The data packet 70 includesa 48 symbol preamble 80 followed by a 440 symbol payload 90. Thepreamble 80 includes pilot symbols, which are specified in advance andknown by the receiver. In general, any number of pilot symbols may belocated anywhere or distributed throughout the data packet 70, but theDVB-RCS standard specifies that pilot symbols are located in a preamble80 of the data packet. These known symbols are typically used for signaldetection, synchronization, and so forth. The payload 90 includes theunknown data of the underlying, substantive data transmission. Thesymbols of the payload 90 may also include substantive data as well aserror checking or other bits used for administration of the system. Inparticular, the payload 90 under the DVB-RCS standard includes“systematic bits” defining substantive data as wells as “parity bits”used for error checking.

As illustrated in FIG. 2, each symbol in the data packet 70 is a phasor,represented by the phasors 92 a-n, in which modulating data is expressedas the symbol phase angle. Under the QPSK data modulation technique,each symbol may have four states (i.e., 0, π/2, π and 3π/2). Thus, eachsymbol may be translated into a two-bit binary representation. For theBSPK data modulation technique, each symbol may have two states (i.e.,0, π) and can therefore be represented by a single binary bit. For thegeneralized MSPK data modulation technique, each symbol may have “M”states and can therefore be represented by “n” binary bits whereM=2^(n). The symbols are generally represented by the designation Z(n),and a sample or measurement of the symbol is generally referred to as“channel observations” represented by the designation Z′(n). That is,the nominal, intended or undistorted baseband data signal is generallyrepresented by the designation Z(n), whereas the actual (distorted orcorrupted) channel observations (for example as distorted or corruptedby the carrier frequency offset) is generally represented by thedesignation Z′(n). The signal processing operation of the signaldistortion estimator can, therefore, be expressed as detectingcandidates for the carrier frequency offset in the channel observationsZ′(n) which are used to reconstruct estimates of the undistorted symbolsequence Z(n).

FIG. 3 is a conceptual drawing of data symbols illustrating the signaldistortion effect of a carrier frequency offset. Specifically, thecarrier frequency offset causes the symbol phase angle to be offset(rotated) slightly from its nominal or expected value. For a one percent(1%) carrier frequency offset, for example, the symbol phase angle isoffset by one percent or 3.6°. A frequency offset accumulates in phaseover time, which causes the phase offset within a data packet toincrease from symbol to symbol. For example, a one percent (1%) carrierfrequency offset will cause the symbol phase angle error to rotatearound a complete 360° cycle every 100 symbols. For a data DVB-RCSpacket of 488 symbols, this translates into 4.88 complete cycles ofphase error over the data packet, which makes the underlying impossibleto demodulate unless the carrier frequency offset is detected andremoved from the input signal.

FIG. 4 is a conceptual drawing of a burst-mode data packet illustratinga measure for carrier frequency offset. A typical baseband data rate forthe data packet 70 is 64 kBaud (i.e., 64,000 symbols per second). For aone percent (1%) carrier frequency offset, this translates into a 640 Hzcarrier frequency offset (i.e., 0.01×64,000=640 Hz). This is the signaldistortion parameter that the signal distortion estimator of the presentinvention is designed to detect for removal by downstream processing bythe JSD. Because this signal distortion parameter may be masked by noisewhen the signal-to-noise ratio decreases, the signal distortionestimator typically provides several candidate values for the signaldistortion parameter, which the decoder tests in order to select thecorrect candidate. In the particular embodiment described in more detailbelow, the carrier frequency offset is the signal distortion parameterfor which the carrier frequency offset estimator produces threecandidate values using a hybrid data-aided and non-data-aided signalprocessing technique. The JSD then selects a most likely candidate basedon an indication of decoding success and decodes the modulating datafrom the channel observations using the selected candidate, as describedin greater detail below.

FIG. 5 is a logic flow diagram illustrating a method for operating ageneralized JSD. In step 502, the JSD obtains channel observationsassociated with an input signal. Step 502 is followed by step 504, inwhich the JSD obtains multiple candidates for a signal distortionindicator associated with the input signal. Step 504 is followed by step506, in which the JSD modifies the channel observations based on thesignal distortion indicators to obtain modified channel observations forseveral processing branches in which each branch corresponds to one ofthe signal distortion indicator. Step 506 is followed by step 508, inwhich the JSD performs initial and additional synchronization on themodified channel observations to obtain corrected channel observationsfor each processing branch. Step 508 is followed by step 510, in whichthe JSD applies partial decoding to the corrected channel observationsto obtain an indication of decoding success for each processing branch.Step 510 is followed by step 512, in which the JSD selects a processingbranch with a most likely candidate signal distortion indicator fordecode processing based on an analysis of the indications of decodingsuccess for the several processing branches. Steps 506 through 512 arealso referred to jointly as “decision aided candidate selection” toindicate that indications of decoding success are used to select acandidate signal distortion indicator for decode processing.

Step 512 is followed by step 514, in which the JSD applies decisionaided synchronization to the input signal by using the selected signaldistortion indicator to synchronize to the input signal. Step 514 isfollowed by step 516, in which the JSD applies a first iteration ofdecode processing to the input signal. Step 516 is followed by step 518,in which the JSD obtains an indication of decoding success for thedecode processing. The JSD then loops through steps 514 through 518 fora sufficient number of iterations to complete the decode processing.Steps 514 through 518 are also referred to jointly as “decision aidedsynchronization and decoding” to indicate an indication of decodingsuccess is used to synchronize to the input signal and decode themodulating data carried by the input signal., FIG. 6 is a logic flowdiagram illustrating a method for operating a particular embodiment theJSD 500 shown on FIG. 7 for phase tracking in a satellite-based systemoperating under the DVB-RCS standard, as described previously withreference to FIGS. 1-4. In step 602, the JSD 500 obtains channelobservations associated with an input signal from the channel observer300 shown on FIG. 7. Step 602 is followed by step 604, in which the JSD500 obtains multiple carrier frequency offset candidates from the hybridcarrier frequency offset estimator 400 shown on FIG. 7. Step 604 isfollowed by step 606, in which the JSD 500 modifies the channelobservations based on the carrier frequency offset candidates to obtainphase modified channel observations for several processing branches inwhich each branch corresponds to one of the carrier frequency offsetcandidates. Step 606 is followed by step 608, in which the JSD 500initializes and synchronizes the phase modified channel observations toobtain phase corrected channel observations for each processing branch.Step 608 is followed by step 610, in which the JSD 500 applies partialdecoding to the phase corrected channel observations to obtain anindication of decoding success for each processing branch. In thisparticular example, a log-likelihood ratio (LLR) produced by the partialdecoders 1204 a-n shown on FIG. 12 provide the indications of decodingsuccess. Step 610 is followed by step 612, in which the JSD 500 selectsa processing branch with a most likely carrier frequency offsetcandidate for decode processing based on an analysis of the LLR valuesfor the several processing branches. In this particular example, theabsolute values of the LLR values corresponding to all of the symbols ofa data packet are averaged and the candidate with the highest averageLLR value is selected. Steps 606 through 612 are also referred tojointly as “decision aided carrier frequency offset estimation” toindicate that indications of decoding success, in this case the LLRvalues, are used to select a carrier frequency offset estimationcandidate for decode processing.

Step 612 is followed by step 614, in which the JSD 500 producescorrected channel observations by applying decision aided phase trackingto the input signal using the select carrier frequency offset candidateto synchronize to modified and initialized carrier observations. Step614 is followed by step 616, in which the JSD 500 applies a firstiteration of decode processing to the corrected channel observations.Step 616 is followed by step 618, in which the JSD 500 obtains anindication of decoding success for the decode processing. In thisembodiment, these are the LLR values produced by the decoder 1504 shownin FIG. 15. The JSD 500 then loops through steps 614 through 618 for asufficient number of iterations to complete the decode processing. Steps614 through 618 are also referred to jointly as “decision aided phasetracking” to indicate that an indication of decoding success, in theembodiment LLR values, are used to synchronize to the input signal anddecode the modulating data carried by the input signal.

FIG. 7 is a block diagram of the return channel receiver 60 within thesatellite communication system 10 shown on FIG. 1. The demodulator 62 inthis particular embodiment includes a channel observer 300 that receivesthe analog input signal from the antenna of the satellite hub 30. Thechannel observer 300 extracts a channel observations 34 [Z′(n)] from theinput signal. As noted previously, the channel observations contains thedata symbols as corrupted by the carrier frequency offset. The channelobserver 300 provides the channel observations 34 to a hybrid frequencyoffset estimator 400 and to the JSD 500. The hybrid frequency offsetestimator 400 also provides multiple carrier frequency offset candidates36 to the JSD 500, which selects a most likely candidate for the carrierfrequency offset to synchronize with the channel observations and decodethis signal to produce decoded bits 38 with increased reliability in alow signal-to-noise environment.

The joint synchronizer and decoder 500 is described in detail below,whereas the hybrid frequency offset estimator 400 is described incommonly owned and concurrently filed U.S. patent application Ser.No.______ entitled “Hybrid Frequency offset estimator,” which isincorporated herein by reference. As noted previously, cooperativeoperation with the hybrid frequency offset estimator 400 is oneparticular application of the joint synchronizer and decoder 500, whichcould alternatively obtain its input candidates from any suitabledevice.

FIG. 8 is a block diagram of the channel observer 300, which is part ofthe hybrid frequency offset estimator 400 shown in FIG. 7. The channelobserver 300 includes a front-end analog processor 310 and a digitalpreliminary processor 320, which are both conventional in design.Briefly described, the front-end analog processor 310 converts theanalog input signal 32, typically a Ku or Ka band signal (e.g., 11 or 20GHz) to an intermediate frequency signal 35 (e.g., 400 MHz) that issuitable for processing in conventional solid state hardware. Thedigital preliminary processor 320 digitizes the intermediate frequencysignal 35 to produce the channel observations 34.

FIG. 9 is a simplified block diagram of the front-end analog processor310 shown on FIG. 8. This processor typically includes a low-noiseamplifier 312 to amplify the input signal 32, a down converter 314 toconvert the input signal to the intermediate frequency, and anintermediate frequency filter 316 to remove noise and produce theintermediate frequency signal 35. These functional blocks, which arewell known in the field of signal processing, will not be furtherdescribed in this specification.

FIG. 10 is a block diagram of the digital preliminary processor 320shown on FIG. 8. This processor includes an analog-to-digital converter322, which converts the intermediate frequency signal 35 to a digitalMF-TDMA signal 42. This signal is passed to a digital demutiplexer 324,which converts the digital MF-TDMA signal 42 into a single-channel TDMAsignal 44. This signal is passed to a pulse shaping filter 326, whichproduces a filtered TDMA signal 46. This signal is passed to a burstdetector and timing estimator 328 that detects the start of data burstsand outputs the timing-adjusted channel observation 34. Again, thesefunctional blocks are well known in the field of signal processing andwill not be further described in this specification.

FIG. 11 is a block diagram of a generalized JSD, which includes “n”parallel processing branches, a first branch associated with a firstsignal distortion indicator (candidate-A), a second branch associatedwith a second signal distortion indicator (candidate-B) and so forth upto an “nth” branch associated with an “nth” signal distortion indicator(candidate-N). The first branch includes a channel modifier 1102 a, asynchronization processor 1104 a, and a partial decoder 1106 a. Thesecond branch includes a channel modifier 1102 b, a synchronizationprocessor 1104 b and a partial decoder 1106 b. The “nth” branch includesa channel modifier 1102 n, a synchronization processor 1104n and apartial decoder 1106 n. Because each branch operates in the same mannerexcept that the associated signal distortion indicator (candidate) isdifferent, only the first branch will be described in detail.

The channel modifier 1102 a receives an associated signal distortionindicator (candidate-A) and the channel observations Z′(n) 34. Thesignal distortion indicator (candidate-A) is a potential indicator ofsignal distortion in the channel observations Z′(n) 34, which typicallymodulates an input carrier signal. The channel modifier 1102 a producesmodified channel observations by removing potential signal distortionfrom the channel observations Z′(n) 34 using the signal distortionindicator (candidate-A). In this manner, the modified channelobservations represent a test of the effectiveness of the signaldistortion indicator (candidate-A) for use in removing signal distortionfrom the channel observations Z′(n) 34. That is, a subsequent comparisonof the success of the several signal distortion indicator candidates inremoving signal distortion from the channel observations Z′(n) 34 isused to select a most likely candidate for removing signal distortionfrom the channel observations Z′(n) 34.

The channel modifier 1102 a passes the modified channel observations tothe synchronization processor 1104 a, which produces corrected channelobservations by synchronizing the modified channel observations.Typically, the synchronization process involves phase tracking, timingtracking, amplitude tracking or another tracking function appropriatefor the modulation technique employed in the channel observations Z′(n)34. The synchronization processor 1104 a then passes the correctedchannel observations to the partial decoder 1106 a, which decodes thecorrected channel observations to obtain decoded data bits and anindication of decoding success. In particular, the partial decoder 1106a is typically a soft-input-soft-output iterative decode processordesigned to perform decode processing through a number of iterations,for example ten iterations. However, the partial decoder 1106 a onlyperforms a small percentage of these iterations, for example one tothree iterations, sufficient to obtain an indication of decodingsuccess. Complete decode processing is not typically performed for eachprocessing branch at this stage.

The partial decoder 1106 a then passes the indication of decodingsuccess for the first processing branch to a branch selector 1108. Thesynchronization processor 1104 a also passes the corrected channelobservations to the branch selector 1108. Each processing branch doesthe same, thereby providing the branch selector 1108 with the indicationof decoding success and the corrected channel observations from eachprocessing branch. The branch selector 1108 then selects a most likelybranch corresponding to the most likely candidate for the signaldistortion indicator based on a comparison of the indications ofdecoding success for the several processing branches.

The candidate selection process described above is referred to as“decision aided candidate selection” to indicate that the indications ofdecoding success for the several processing branches have been used toselect a most likely processing branch and associated signal distortionindicator. Once the most likely processing branch has been selected, thebranch selector 1108 passes the corrected channel observations and theindication of decoding success for the selected branch to an iterativeprocessor 1110, which decodes the corrected channel observations toobtain the encoded data bits.

It should be noted that each individual processing branch shown in FIG.11 is conventional in that this type of single-branch synchronizationand decode processing is well known in the field of signal processing.On the other hand, the use of multiple branches associated withdifferent signal distortion indicators (candidates) and the selection ofone of the branches for decode processing based on an indication ofdecoding success for the several branches (i.e., decision aidedcandidate selection) is an improvement associated with the JSD accordingto present invention. A specific example of a JSD implementing decisionaided candidate selection and decision aided synchronization anddecoding is described in detail below with reference to a particularexample configured to implement phase tracking within a return channelreceiver in a DVB-RCS system using turbo coding and quadrature phaseshift key (QPSK) data modulation.

The iterative processor 1110 typically implements asoft-input-soft-output decoding algorithm, such as a turbo decodingalgorithm, several types of which are well known in the field of signalprocessing. Preferably, the iterative processor 1110 also implementsdecision aided synchronization and decoding, in which an indication ofdecoding success is used in the synchronization process. This type ofdecision aided synchronization and decoding is not conventional, andrepresents an improvement associated with the JSD according to presentinvention.

FIG. 15 is a block diagram of the iterative processor 1110. Thisprocessor includes a synchronization processor 1502 and a decoder 1504.In the first iteration of the iterative processor, indications ofdecoder success are passed from the branch selector 1108 to thesynchronization processor 1502, which in turn computes corrected channelobservations based on the indication of decoder success and the modifiedand initialized channel observations. The operations within thesynchronization processor 1502 are not necessarily the same as thosewithin the synchronization processor 1104 a-n.

The corrected channel observations are passed to the decoder 1504, whichcomputes the decoded bits 38 as well as indication of decoder success1506. The decoder 1504 is typically a soft-input-soft-output iterativedecode processor designed to perform decode processing through a numberof iterations, for example ten iterations. However, the decoder 1504typically only performs a small fraction of these iterations, forexample one to three iterations, sufficient to obtain an indication ofdecoding success 1506.

In second and subsequent iterations of the iterative processor, theindication of decoder success 1506 is passed to the synchronizationprocessor 1502. FIG. 12 is a block diagram of the JSD 500 shown in FIG.7, which is configured to implement decision aided carrier frequencyoffset estimation (i.e., a example of decision aided candidateselection) and decision aided phase tracking (i.e., an example ofdecision aided synchronization and decoding) within a burst-mode DVB-RCSsystem. The JSD 500 typically applies the signal processing methodologydescribed below on a per-packet bases. That is, the JSD 500 appliesdecision aided candidate selection and decision aided synchronization,as described below, to the symbols of an entire packet. The process isthen repeated for each packet processed by the JSD 500.

The JSD 500 includes three parallel processing branches, a first branchassociated with a first carrier frequency offset (candidate-A), a secondbranch associated with a second carrier frequency offset (candidate-B)and a third branch associated with a third carrier frequency offset(candidate-C). The first branch includes a frequency corrector 1202 a,an initial phase estimator and corrector 1204 a, a phase tracking module1205 a, and a partial decoder 1206 a. The second branch includes afrequency corrector 1202 b, an initial phase estimator and corrector1204 b, a phase tracking module 1205 b, and a partial decoder 1206 b.The third branch includes a frequency corrector 1202 c, an initial phaseestimator and corrector 1204 c, a phase tracking module 1205 c, and apartial decoder 1206 c. Because each branch operates in the same mannerexcept that the associated carrier frequency offset candidate isdifferent, only the first branch will be described in detail.

The frequency corrector 1202 a receives an associated carrier frequencyoffset (candidate-A) from the hybrid carrier frequency estimator 300shown on FIG. 7. The frequency corrector 1202 a also receives thechannel observations Z′(n) .34 from the channel observer 300 shown onFIG. 7. The carrier frequency offset (candidate-A) is a potentialindicator of carrier frequency offset in the channel observations Z′(n)34. As described previously with reference to FIGS. 1-4, the channelobservations modulate an input signal 32 with encoded data using QPSKmodulation. The carrier frequency offset is a source of signaldistortion that must be removed before the encoded data can bedemodulated from the channel observations.

The frequency corrector 1202 a produces modified channel observations byremoving the carrier frequency offset (candidate-A) from the channelobservations Z′(n) 34. In this manner, the modified channel observationsrepresent a test of the effectiveness of “candidate-A” for use inremoving carrier frequency offset from the channel observations Z′(n)34. That is, a subsequent comparison of the success of the severalcandidates in removing carrier frequency offset from the channelobservations Z′(n) 34 is used to identify a most likely candidate thatcorrectly represents the carrier frequency offset in the channelobservations Z′(n) 34. The frequency corrector 1202 a is well known inthe field of signal processing, and will not be descried further in thisspecification.

The channel modifier 1202 a passes the modified channel observations tothe initial phase estimator and corrector 1204 a, which adjusts thephase of the modified channel observations in accordance with an initialphase estimate. In other words, the tracking process is initialized bythe initial phase estimator 1204 a. This module ensures that the channelobservations are initialized in such a way that the phase offset is nearzero at the start of the sequence of channel observations Z′(n). Theinitial phase estimator and corrector 1204 a is well known in the fieldof signal processing, and will not be descried further in thisspecification. The initial phase estimator and corrector 1204 a passesthe initialized and modified channel observations to the phase trackingmodule 1205 a, which tracks the phase of the initialized and modifiedchannel observations to produce phase corrected channel observations.Although the phase tracking module 1205 a is well known in the field ofsignal processing, a typical block diagram for this module is shown inFIG. 13 for the purpose of distinguishing this conventional module fromthe decision aided phase tracking module 1604 shown in FIGS. 16 and 17.

The phase tracking module 1205 a then passes the phase corrected channelobservations to the partial decoder 1206 a, which demodulates the phasecorrected channel observations to obtain decoded data bits and anindication of decoding success. In this instance, the indication ofdecoding success is represented by log-likelihood ratios (LLRs) producedby the partial decoder 1206 a using a soft-input-soft-output turbodecoding algorithm. Specifically, the partial decoder 1206 a producesone LLR for each symbol modulating the channel observations Z′(n) 34,and the LLRs for a complete data packet are processed together on aper-packet basis. It should be appreciated that the partial decoder 1206a is an. iterative decode processor designed to perform a turbo decodingalgorithm through a number of iterations, for example ten iterations.However, the partial decoder 1206 a only performs a small percentage ofthese iterations, for example one to three iterations, sufficient toobtain suitable LLRs for the data packet being processed. Completedecode processing is not typically performed for each processing branchat this stage.

The partial decoder 1206 a then passes the LLRs for the first processingbranch to a branch selector 1208. The phase tracking module 1204 a alsopasses the phase corrected channel observations to the branch selector1208. Each processing branch does the same, thereby providing the branchselector 1208 with the LLRs and the phase corrected channel observationsfrom each: processing branch. The branch selector 1208 then selects amost likely branch corresponding to the most likely carrier frequencyoffset candidate based on a comparison of the LLRs for the severalprocessing branches. Specifically, the absolute values of the LLRs forthe data packet being processed are averaged, and the branch selector1208 selects the carrier frequency offset candidate producing thelargest averaged absolute values of the LLRs for the packet as the mostlikely candidate.

The candidate selection process described above is referred to as“decision aided carrier frequency estimation” to indicate that theindications of decoding success for the several processing branches havebeen used to select a most likely processing branch and associatedcarrier frequency offset. Once the most likely processing branch hasbeen selected, the branch selector 1208 passes the phase correctedchannel observations and the LLRs for the selected branch to aniterative processor 1210, which decodes the phase corrected channelobservations to obtain the encoded data bits. The iterative processor1210 is described in detail below with reference to FIGS. 14-17.

FIG. 13 is a block diagram of the prior art phase tracking module 1205a-c shown on FIG. 12. The phase tracking module 1205 a-c includes a harddecision module 1302 that assigns a “hard value” corresponding to anominal QPSK phase orientation to each symbol in the modified andinitialized channel-observations received from the initial phaseestimator and corrector 1204 shown on FIG. 12. Although there may beuncertainly in the hard decision at this point, a specific hard decisionis made based on the best information currently available (i.e., at thephase corrected channel observation) to facilitate the phase trackingprocess.

The hard decision module 1302 passes the hard decision QPSK phaseorientation to a complex conjugate module 1304, which computes thecomplex conjugate of the hard decision QPSK phase orientation. Themodified and initialized channel observations are then multiplied by thecomplex conjugate of the hard decision QPSK phase orientation. Thisremoves the QPSK modulation from the modified and initialized channelobservations, leaving only a phasor representing the phase differencebetween the modified and initialized channel observations and the harddecision QPSK phase orientation. This phase difference represents adetected phase error in the modified and initialized channelobservations. This detected phase error is then fed to an argumentprocessor 1306, which determines the phase angle of the detected phaseerror. The phase angle of the detected phase error is then passed to alow pass filter 1308 to remove noise. The result is a phase trackingparameter (i.e., a particular example of a synchronization parameter)representing the phase angle of the detected phase error in the modifiedand initialized channel observations is then provided as an output ofthe phase tracking module 1205 a.

This phase tracking parameter is also fed back for error correction.Specifically, a phasor corresponding to a negative or cancelingcomplement of the phase angle of the detected phase error is determinedby a sin/cos lookup table 1310, and this phasor is then multiplied bythe modified and initialized channel observations to remove the detectedphase error from the modified and initialized channel observations. Thismultiplication process results in the phase corrected channelobservation, which is also an output of the phase tracking module 1205a. As noted previously, this phase tracking process well known in thefield of signal processing.

FIG. 14 is a block diagram of the prior art partial decoder function1206 a-c for the turbo code of the DVB-RCS standard. The decoderimplements an iterative soft-input-soft-output algorithm, typically aturbo decoding algorithm, which is well known in the signal processingindustry. The phase corrected channel observations received from thephase tracking module 1205 a are mapped into bit-wise elements forsystematic and parity bits, respectively, by the symbol-to-bit mappingfunction 1406. The bit-wise elements for the systematic bits and for theparity bits of the first component of the composite code are forwardedto the decoder 1408 for the first component code. The bit-wisecomponents for the parity bits of the second component of the compositecode are forwarded to the decoder 1412 for the second component code.The bit-wise components for the systematic bits are also forwarded thedecoder 1412 for the second component code; however, these are passedthrough interleaver 1410 b in order to match the order in which theywere encoded.

In the first iteration of the decoder 1206 a-c, pre-set LLR valuesrepresenting the absence of prior knowledge about the informationcontained in the packet are passed to the decoder for the firstcomponent code 1408, through de-interleaver 1404. The decoder for thefirst component code 1408 computes improved LLR values for thesystematic bits of the packet, using a computational technique wellknown in the signal processing industry. The improved LLR values arethen passed as input to the decoder for the second component code 1412,but are passed through interleaver 1410 a in order to match the encodingorder of the second component code. The decoder for the second componentcode 1412 computes improved log-likelihood ratios for the systematicbits of the packet, using a computational technique well known in thesignal processing industry.

The LLRs computed by the second decoder 1412 are output to the branchselector 1208. They are also provided as input to the decoder for thefirst component code 1408 in the subsequent iteration. The input to thefirst decoder 1408 is passed through de-interleaver 1404, in order tomatch the order of encoding in the first component code. The number ofiterations performed for each decoder 1206 a-c is anapplication-dependent design parameter.

FIG. 15 is a block diagram of the iterative processor 1208, which isshown in FIG. 12. The iterative processor 1208 includes a decoderassisted phase tracker 1502 and a decoder 1504. The decoder assistedphase tracker 1502 is a type of synchronization processor configured toimplement phase tracking. The decoder assisted phase tracker 1502produces phase corrected channel observations based on modified andinitialized channel observations and LLR values for the data packetbeing processed. The structure of the decoder assisted phase tracker1502 and the functions implemented to compute the phase correctedchannel observations are described in greater detail with reference toFIG. 16.

The decoder 1504 processes the corrected channel observations and theLLR values to extract the decoded bits 38 through an iterative decodingprocess, which is similar to the iterative process described withreference to FIG. 14. The important difference is that the LLR valuesproduced by the decoder 1504 are also fed back to the decoder assistedphase tracker 1502 for use in decision aided phase tracking. Inaddition, the feedback loop 1506 includes a switch that feeds the LLRvalues from the partial decoding stage to the decoder assisted phasetracker 1502 for the first iteration. After that initial iteration, theswitch feeds the LLR values from the decoder 1504 to the decoderassisted phase tracker 1502 to implement the feedback loop.

FIG. 16 is a block diagram of the decoder assisted phase tracker 1502shown in FIG. 15. The decoder assisted phase tracker 1502 includes asoft decision aided phase tracker 1602 and a decision aided phasetracking module 1604. The soft decision aided phase tracker 1602 usesindications of decoding success, in this case LLR values, in the phasetracking process for symbols carrying the systematic bits. The decoderassisted phase tracker 1502 receives the LLR values 1506 from thedecoder 1504 and the initialized and modified channel observations asinputs, and produces phase corrected channel observations as an output.

More specifically, the decoder assisted phase tracker 1502 includes aswitch 1603 that supplies symbols from the initialized and modifiedchannel observations carrying systematic bits to the soft decision aidedphase tracker 1602. The switch 1603 also supplies symbols from theinitialized and modified channel observations carrying parity bits tothe decision aided phase tracking module 1604. The decoder assistedphase tracker 1502 also includes a switch 1605 that supplies correctedchannel observations for the systematic bits from the soft decisionaided phase tracker 1602 as an output. The switch 1605 also suppliescorrected channel observations for the parity bits from the decisionaided phase tracking module 1604 as an output. In other words, thecorrected channel observations for the systematic bits are produced bythe soft decision aided phase tracker 1602, whereas the correctedchannel observations for the parity bits are produced by the decisionaided phase tracking module 1604.

In addition, the soft decision aided phase tracker 1602 includes a harddecision and quality estimate module 1606 that receives the LLR values1506 from the decoder 1504. The hard decision and quality estimatemodule 1606 assigns a “hard value” corresponding to QPSK phaseorientation to each symbol based on the LLR values. Although there maybe uncertainly in the hard decision at this point, a specific harddecision is made based on the best information currently available(i.e., at the present iteration) to facilitate the phase trackingprocess. The LLR value also indicates a likelihood or quality estimatefor this hard decision. As noted previously, the LLR values typicallyincrease with successive iterations indicating increasing likelihood orquality of the hard decisions.

The hard decision and quality estimate are then fed to a modulationremoval module 1608, which also receives the initialized and modifiedchannel observations carrying the systematic bits. The modulationremoval module 1608 then removes the modulation from the initialized andmodified channel observations through multiplication with the complexconjugate of the associated hard decision. The result of this process isa phase error, which the modulation removal module 1608 scales by theassociated quality estimate to produce a modulation-free resultantsignal. This signal is then passed to a sliding average module 1610,which computes a sliding average of the modulation-free resultantsignal, typically over a block of 50 symbols, to produce a phasetracking parameter representative of the phase error in the initializedand modified channel observations for the systematic bits. The phasetracking parameter is then passed to a phase corrector 1612, whichremoves the phase error represented by the phase tracking parameter fromthe initialized and modified channel observations for the systematicbits to compute the phase corrected channel observations for thesystematic bits.

As noted previously, the symbols carrying the parity bits are passed tothe decision aided phase tracking module 1604. The phase trackingparameter is also fed from the sliding average module 1610 of the softdecision aided phase tracker 1602 to the decision aided phase trackingmodule 1604. The decision aided phase tracking module 1604 also includesa switch 1607 that switches between the phase tracking parameterreceived from the soft decision aided phase tracker 1602 for the firstsymbol and an internally-generated phase tracking parameter forsubsequent symbols.

FIG. 17 is a block diagram of the phase tracking module 1604, which issimilar to the prior art phase tracking module 1205 a-c describedpreviously except for the presence of the switch 1607, which is used toswitch between the phase tracking parameter received from the softdecision aided phase tracker 1602 for its first iteration and aninternally-generated phase tracking parameter for subsequent iterations.

In view of the foregoing, it will be appreciated that present inventionprovides an improved joint synchronizer and decoder that is configuredto implement decision aided candidate selection and decision aidedsynchronization and decoding. More specifically, the invention select acarrier frequency offset by selecting among a number of candidates forthis parameter based on an indication of decoding success. The inventionmay also implement decision aided phase tracking based on an indicationof decoding success. It should be understood that the foregoing relatesonly to the exemplary embodiments of the present invention, and thatnumerous changes may be made therein without departing from the spiritand scope of the invention as defined by the following claims.

1. A joint decoder and synchronizer for operating on an input signalmodulated with encoded data, comprising: a plurality of channelobservation modifiers, each configured for receiving an associatedcandidate signal distortion indicator and channel observationsassociated with the input signal, each channel observation modifierfurther operable for producing modified channel observations comprisingthe encoded data based on the associated candidate signal distortionindicator and the channel observations; a synchronization processorassociated with each channel observation modifier, each synchronizationprocessor operative for receiving modified channel observations from itsassociated channel observation modifier, synchronizing to the modifiedchannel observations received from its associated channel observationmodifier, and producing corrected channel observations; a partialdecoder associated with each synchronization processor, each partialdecoder operative for partially decoding the encoded data embeddedwithin the corrected channel observations received from its associatedsynchronization processor and producing an associated indication ofdecoding success; and a branch selector operative for selecting a signaldistortion indicator for decode processing based on a comparison of theindications of decoding success received from the partial decoders. 2.The joint decoder and synchronizer of claim 1, wherein the input signalcomprises burst-mode multi-frequency time division multiplexed (MF-TDMA)data in which the data signal is encoded using a quadrature phase shiftkey (QPSK) modulation technique in accordance with the digital videobroadcast return channel via satellite (DVB-RCS) standard.
 3. The signalprocessor of claim 1, further comprising a channel observer forprocessing the input signal to produce the channel observations.
 4. Thesignal processor of claim 3, wherein the channel observer furthercomprises a front-end analog processor operable for amplifying the inputsignal, down-converting the amplified input signal to obtain anintermediate frequency signal, and filtering the intermediate frequencysignal.
 5. The signal processor of claim 3, wherein the channel observerfurther comprises a digital preliminary processor operable fordigitizing a representation of the input signal to obtain a digitalMF-TDMA signal, demultiplexing the MF-TDMA signal to obtain aconcatenated single-channel TDMA signal, and detecting burst-mode datapackets within the single-channel TDMA signal to obtain the channelobservations.
 6. The joint decoder and synchronizer of claim 1, whereinthe indication of decoding success comprises a log likelihood ratioproduced by the partial decoder.
 7. The joint decoder and synchronizerof claim 1, wherein: the encoded data comprises a plurality of symbols;the indication of decoding success comprises a log likelihood ratioproduced by the partial decoder for each symbol; and the branch selectorselects a signal distortion indicator based on the highest average loglikelihood ratio for each symbol over a common range of the symbols. 8.The joint decoder and synchronizer of claim 1, wherein the channelobservations comprise symbols having phase components modulated by theencoded data.
 9. A joint decoder and synchronizer for operating on aninput signal modulated with encoded data, having an iterative processorcomprising: a synchronization processor operative for receiving andsynchronizing to channel observations associated with the input signal;a decoder operative for decoding the synchronized channel observationsreceived from the synchronization processor and producing an associatedindication of decoding success; a feedback loop supplying the indicationof decoding success to the synchronization processor; and thesynchronization processor further operable for adjusting itssynchronization to the channel observations based on the indication ofdecoding success.
 10. The joint decoder and synchronizer of claim 9,wherein the indication of decoding success comprises a log likelihoodratio produced by the decoder.
 11. The joint decoder and synchronizer ofclaim 9, wherein the synchronization processor further comprises a softdecision aided synchronization processor operative for computing asynchronization parameter based on the indication of decoding success,the soft decision aided synchronization processor further operative forcomputing a corrected channel observation based on the input channelobservation and the synchronization parameter.
 12. A joint decoder andsynchronizer for operating on an input signal modulated with encodeddata, comprising: a plurality of frequency correctors, each configuredfor receiving an associated carrier frequency offset candidate andchannel observations associated with the input signal, each frequencycorrector further operable for producing modified channel observationscomprising the encoded data based on the associated carrier frequencyoffset candidate and the channel observations; an initial phaseestimator and corrector configured to initialize the modified channelobservations to an initial phase; a phase tracking module associatedwith each frequency corrector, each phase tracking module operative forsynchronizing to the initialized and modified channel observationsreceived from its associated initial phase estimator and corrector toproduce phase corrected channel observations; a partial decoderassociated with each phase tracking module, each partial decoderoperative for partially decoding the encoded data embedded within thephase corrected channel observations received from its associated phasetracking module and producing an associated indication of decodingsuccess; and a branch selector operative for selecting a carrierfrequency offset candidate for decode processing based on a comparisonof the indications of decoding success received from the partialdecoders.
 13. The joint decoder and synchronizer of claim 12, whereinthe input signal comprises burst-mode multi-frequency time divisionmultiplexed (MF-TDMA) data in which the data signal is encoded using aquadrature phase shift key (QPSK) modulation technique in accordancewith the digital video broadcast return channel via satellite (DVB-RCS)standard.
 14. The signal processor of claim 13, further comprising achannel observer for processing the input signal to produce the channelobservations.
 15. The signal processor of claim 14, wherein the channelobserver further comprises a front-end analog processor operable foramplifying the input signal, down-converting the amplified input signalto obtain an intermediate frequency signal, and filtering theintermediate frequency signal.
 16. The signal processor of claim 15,wherein the channel observer further comprises a digital preliminaryprocessor operable for digitizing a representation of the input signalto obtain a digital MF-TDMA signal, demultiplexing the MF-TDMA signal toobtain a concatenated single-channel TDMA signal, and detectingburst-mode data packets within the single-channel TDMA signal to obtainthe channel observations.
 17. The joint decoder and synchronizer ofclaim 16, wherein the indication of decoding success comprises a loglikelihood ratio produced by the partial decoder.
 18. The joint decoderand synchronizer of claim 17, wherein: the encoded data comprises aplurality of symbols; the indication of decoding success comprises a loglikelihood ratio produced by the partial decoder for each symbol; andthe branch selector selects a signal distortion indicator based on thehighest average log likelihood ratio for each symbol over a common rangeof the symbols.
 19. The joint decoder and synchronizer of claim 18,wherein the channel observations comprise symbols having phasecomponents modulated by the encoded data.
 20. A joint decoder andsynchronizer for operating on an input signal modulated with encodeddata, including an iterative processor comprising: a decoder assistedphase tracker operative for receiving and synchronizing to channelobservations associated with the input signal and producing phasecorrected channel observations; a decoder operative for decoding thephase corrected channel observations received from the decoder assistedphase tracker and producing an associated indication of decodingsuccess; and a feedback loop supplying the indication of decodingsuccess to the decoder assisted phase tracker; and the decoder assistedphase tracker further operable for adjusting its synchronization to thechannel observations based on the indication of decoding success. 21.The joint decoder and synchronizer of claim 20, wherein the indicationof decoding success comprises a log likelihood ratio produced by thedecoder.
 22. The joint decoder and synchronizer of claim 20, wherein theencoded data define systematic bits representing unknown data and paritybits derived from the systematic bits for integrity checking, andwherein the decoder assisted phase tracker further comprises: a softdecision aided phase tracker operative for computing a phase trackingparameter based on the systematic bits and the indication of decodingsuccess, the soft decision aided phase tracker further operable forcomputing corrected channel observations based on the systematic bitsand the phase tracking parameter; a phase tracking module operative forreceiving the phase tracking parameter from the soft decision aidedphase tracker and computing a corrected channel observations based onthe parity bits and the phase tracking parameter; and a switch operativefor feeding the systematic bits to the soft decision aided phasetracker, and for feeding the parity bits to the phase tracking module.23. The joint decoder and synchronizer of claim 22, wherein the phasetracking module further comprises a feedback loop comprising a phasetracking parameter associated with the corrected channel observations.24. The joint decoder and synchronizer of claim 22, wherein the softdecision aided phase tracker computes the phase tracking parameter byremoving modulation from the channel observations, obtaining a resultantsignal by weighting the modulation-free channel observations by arepresentation of the indication of decoding success, and computing asliding average of the resultant signal.